You can only write off things that you can't use for personal use.
Continuing education and recertification, yes. Workout-specific music bought from a workout music company, yes. Clothes, unless it's a specific uniform for that gym, no. "Regular" music that you edit and compile yourself, no.
My gym reimburses instructors for 50% of continuing education up to $200 a year, and a flat rate of $25 per quarter for music. They host a CPR re-certification periodically at no cost to us (I have to do that at 7 a.m. tomorrow, eccch.) We are not reimbursed for certification/recertification costs.